1. Field of the Invention
The present invention relates to a plasma processing apparatus for performing a predetermined process using a plasma.
2. Description of the Related Art
In the manufacture of, for example, a semiconductor integrated circuit, plasma is utilized in the steps of ashing, etching, CVD and sputtering treatments in order to promote the ionization of a processing gas, the chemical reaction, etc. It was customary in the past to use in many cases a parallel plate type plasma apparatus using a high frequency (RF) energy as a means for generating a plasma. Recently, proposed is a high frequency induction type plasma processing apparatus using a spiral antenna because the plasma processing apparatus of this type permits a desirable energy density distribution of the plasma, makes it possible to control highly accurately the bias potential between the plasma and the susceptor, and is effective for diminishing the contamination with the heavy metal coming from the electrode. As described in, for example, European Patent Laid-Open Specification No. 379828, the high frequency induction type plasma processing apparatus comprises a processing chamber and a wafer-supporting plate positioned within the processing chamber. In general, the upper wall portion, which is positioned to face the wafer-supporting plate, of the processing chamber is formed of an insulating material such as a silica glass. Also, a spiral antenna is fixed to the outer wall surface of the insulating region of the processing chamber. A high frequency current is allowed to flow through the antenna so as to generate a high frequency electromagnetic field. The electrons flowing within the region of the electromagnetic field are allowed to collide against neutral particles within the processing gas so as to ionize the gas and, thus, to generate a plasma.
In the high frequency induction type plasma processing apparatus, a plasma is formed within the inner space of the processing chamber right under the spiral antenna. Concerning the density distribution of the plasma thus formed relative to the intensity of the electric field, the highest plasma density is formed about midway between the center and the outermost region in the radial direction of the spiral antenna, and the plasma density is gradually lowered toward the center and toward the outermost region of the spiral antenna. In other words, the plasma density is uneven in the radial direction of the spiral antenna. The plasma of the uneven distribution in the radial direction is diffused from the higher density region toward the lower density region, with the result that the plasma density is made considerably uniform near a semiconductor region positioned below the plasma-forming region.
In the conventional plasma processing apparatus of this type, however, the plasma diffusion in the radial direction tends to cause the plasma density in the central region of the semiconductor wafer to be higher than in the outer peripheral region of the wafer, leaving room for further improvement in the uniformity and reproducibility of the plasma processing.